1. Field of the Invention
The present invention relates to a webbing retractor which structures a seat belt device for restraining, by an elongated, strip-shaped webbing belt, the body of a vehicle occupant who is seated in a seat of a vehicle or the like.
2. Description of the Related Art
A seat belt device which restrains, by an elongated, strip-shaped webbing belt, the body of a vehicle occupant who is seated in a seat of a vehicle, is equipped with a webbing retractor which is fixed to the vehicle body at the side of the seat. The webbing retractor has a spool (take-up shaft) whose axial direction runs, for example, substantially along the longitudinal direction of the vehicle. The proximal end side, in the longitudinal direction, of the webbing belt is anchored at the spool. The spool can take the webbing belt up in the form of a roll around the outer peripheral portion of the spool. When the seat belt device is not being used, the webbing belt can be accommodated in a state of being taken-up on the outer peripheral portion of the spool.
An urging member, such as a spiral spring or the like which urges the spool in a take-up direction in which the spool takes up the webbing belt, is provided at the webbing retractor. Due to the urging force of this urging member, the webbing belt is taken up and accommodated. In the state in which the webbing belt is applied to the body of a vehicle occupant, slack or the like of the webbing belt is eliminated by the urging force of the urging member.
On the other hand, a mechanism has been conceived in which, by taking-up a predetermined amount of the webbing belt onto the take-up shaft in a state of rapid deceleration of the vehicle or the like, the slight looseness known as “slack” or the like can be eliminated, and the force for restraining the body of the vehicle occupant by the webbing belt is increased such that the body of the vehicle occupant is held even more reliably. In such a mechanism, generally, the state of a rapid deceleration of the vehicle is sensed by an acceleration sensor, and the take-up shaft is forcibly rotated in the take-up direction on the basis of an electric signal from the acceleration sensor.
In contrast, a structure has been thought of in which the distance to another vehicle or an obstacle which is ahead is detected by a distance sensor or the like. When the distance to the vehicle or the obstacle which is ahead is less than a given value, a motor is operated, and the take-up shaft is rotated in the take-up direction by the torque of the motor.
Even in such a structure in which the take-up shaft is rotated in the take-up direction by the torque of a motor when the distance to a vehicle or an obstacle which is ahead is less than a given value, usually, the spool is rotated in the take-up direction by the urging force of the aforementioned urging member such as a spiral spring or the like. When the webbing belt is pulled out, the spool is rotated against the urging force of the urging member such as a spiral spring or the like. Thus, a clutch is provided between the motor and the spool so that the rotation of the spool at the time of usual taking-up and pulling-out of the webbing belt is not transmitted to the output shaft of the motor. Only in cases in which the motor is operated is the output shaft of the motor mechanically connected to the spool.
As a clutch mechanism used in such a structure, there is a structure in which an inertial mass body called an inertial plate is provided so as to be freely rotatable around the axis of the spool. In such a clutch mechanism, the inertial plate is provided so as to be able to rotate relative to both a driven shaft which is integral with the spool and a rotating body of a prime mover which rotates by receiving the rotation of the output shaft of the motor.
However, the other end of an urging member, such as a compression coil spring or the like whose one end is directly or indirectly engaged with the rotating body of the prime mover, is engaged with the inertial plate. When the rotating body of the prime mover rotates due to the torque of the motor, and, accompanying this rotation, the compression coil spring attempts to rotate, the urging force of the compression coil spring rotates the inertial plate.
However, the inertial plate attempts to maintain a stopped state by the inertia thereof. Thus, for example, when the rotating body of the prime mover suddenly rotates, relative rotation arises between the rotating body of the prime mover and the inertial plate which is attempting to maintain the stopped state by inertia. Interlockingly with this relative rotation, a connecting member, such as a pawl or the like which rotates together with the rotating body of the prime mover, is moved, and the connecting member is made to engage with the driven shaft. The torque of the motor is transmitted to the driven shaft, and consequently, to the spool.
However, in a structure using such an inertial plate, there is the condition that the rotating body of the prime mover must be rotated suddenly by the torque of the motor.
In order to satisfy such a condition, the rotating body of the prime mover and the output shaft of the motor must be directly connected without the torque of the motor being decelerated by a reduction gear or the like.
However, in the case of such a structure, if the clutch mechanism does not have a reduction mechanism, the rotation of the rotating body of the prime mover is transmitted to the driven shaft as well without being decelerated. Thus, the taking-up and the like of the webbing belt are carried out excessively rapidly.